Source: National Oceanic and Atmospheric Administration.
Adapted with permission.
In the water cycle, there are two ways water moves from the ground to the atmosphere: evaporation and transpiration. During evaporation, water changes from a liquid to a gas state. Transpiration is basically evaporation of water from plant leaves. Transpiration accounts for about 10 percent of the moisture in the atmosphere — with oceans, seas, and other bodies of water providing nearly all the rest.
During a growing season, a leaf transpires many times more water than its own weight. An acre of corn transpires 3,000–4,000 gallons (11,400–15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year.
The amount of water that plants transpire varies greatly from place to place and over time. There are a number of factors that determine transpiration rates, including temperature, relative humidity, wind, and type of plant. Learn more at weather.gov/jetstream/hydro.
In this activity, you can observe transpiration as water is moved from a plant to the atmosphere. Plant transpiration is usually an invisible process, as the water exiting from the leaves evaporates quickly and the water molecules are very small. The temperature inside the bag will increase as it is heated by the sun. However, the water vapor will condense back into water as it comes in contact with the bag’s surface.
• Living plant with green leaves (trees and bushes work best, another option is an indoor plant with a strong stem)
• Clear plastic bag (freezer zipper bags work well)
• Yarn or twine
• Small graduated cylinder
• Optional: rock or weight
Note: This activity works better on a warm, sunny day or with a plant in a warm, sunny window.
- Place a clear plastic bag around a branch or stem on a tree or bush. If the bag has corners, make sure one corner is pointing down.
- Tie the open end of the bag with yarn. Make sure there are no air leaks.
- Optional: Tie a rock or weight to the bottom of the bag, or tie the bottom of the bag to another branch, so that the bag forms a collection point for the water.
- Observe the plant every hour for 3–6 hours and make note of any condensation in the bag and pooling at the bottom.
- At the end of the activity, cut a small hole in the bottom of the bag with scissors. Collect the water and measure how much was transpired during the activity.
- Develop some ideas to explain your results.